cfg80211: disable CSA for all drivers

The channel switch announcement code has some major locking problems
which can cause a deadlock in worst case. A series of fixes has been
proposed, but these are non-trivial and need to be tested first.
Therefore disable CSA completely for 3.13.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>